The invention relates to spatial/shading error correction systems, and particularly to automatic means for accurately measuring shading and registration errors via digital processing and storage techniques.
In television camera systems employing multiple pickup tubes, various spatial and shading error corrector circuits of varying complexity are used to detect any geometric scan errors due to time variations in the scan across a pickup tube, registration errors due to positional differences of the scan between tubes, and black and white shading errors, ie, video baseline drift and video level variations due to non-uniform output from the pickup tubes. In such systems, it is assumed that the geometric and registration (ie, spatial) scan errors may be corrected by adding a sufficient number of waveshapes such as sawtooth, parabola, etc, waveforms to the basic horizontal and/or vertical deflection waveforms. However, all spatial errors cannot be eliminated using the above techniques, since the scan errors only approximate the two orders of sawtooth and parabola waveforms commonly used for scan correction.
In addition, such corrector systems generally employ analog sources such as potentiometers, integrators, capacitors, etc, remotely-located from the camera head and thus coupled thereto via multiplexed parallel conductors in a cable. The analog error signals necessarily are encoded, multiplexed and decoded, thereby requiring considerable analog circuitry, which generates considerable drift and thus stability problems. The potentiometers continually must be read and re-adjusted, whereby the camera head is continually dependent upon the signals from the remotely-located system.
Typical of such prior art analog error detection and correction systems are those found in the Ampex Corporation broadcast color cameras models BCC-1, BCC-10, described in the Ampex "Service Data Package" Manual No. 1809326-01, Ampex Corporation, Redwood City, Calif.
A more recent and improved spatial and shading error measurement and corrector system is that of co-pending application to F. Morrison, et al, Ser. No. 124,370 filed Feb. 25, 1980, now U.S. Pat. No. 4,285,004, and assigned to the same assignee as this application. Here an improved analog error measurement system is employed to automatically detect existing spatial and shading errors in a camera system during a camera setup mode of operation, whereupon the errors are supplied to the camera head where they are digitally stored for subsequent use by the error correction circuitry thereof during the camera operating mode, independently of the error measurement circuitry. The system measures the spatial errors by providing optical and electronic test patterns. The optical test pattern contains a selected pattern of horizontal and vertical black and white lines corresponding to the electronic test pattern horizontal and vertical frequencies. The video signals of a master (green) pickup tube generated via the optical test pattern are compared with the electronic test pattern signal to provide master tube errors. Then the remaining (red/blue) slave tubes are compared with the master tube to provide the red/blue tube errors. Black and white shading errors are measured by comparing the amplitude of the video signal against selected black and white D.C. levels taken from the test pattern, with and without a cap on the camera lens.
The above analog error measurement system has to cope with the usual analog shortcomings; ie, video baseline drift and offset, use of a limiter for zero detection which eliminates all amplitude information of the video signal etc.